Biennial bearing and crop load

Floral initiation happens in early summer, roughly six to ten weeks after full bloom, in buds that will not open until the following spring. Whether a bud becomes a flower bud or stays vegetative is decided then, and it is decided under the influence of the crop currently on the tree.

Two influences work in the same direction. The first is competition for carbohydrate: a heavy crop is a large sink, and buds initiating flowers are competing with it for the same photosynthate. The second is hormonal, and it is the more decisive: gibberellins exported by developing seeds inhibit floral initiation in nearby buds. A tree carrying many well-seeded fruit is therefore actively signalled not to set flower buds for next year.

Once established, the cycle is self-reinforcing. A light year means little inhibition and abundant initiation, hence a heavy year; a heavy year means strong inhibition, hence a light one. Anything that removes a crop — a spring frost, a hailstorm, a failed pollination — starts the cycle, and an orchard can be locked into it for the rest of its life.

Part of the answer is genetic: the tendency varies strongly between cultivars, and several cultivars long established in cider orchards are recorded as strongly biennial. Yarlington Mill and Tremlett’s Bitter are among the English bittersweets most often described this way, while Michelin and Dabinett are valued precisely for cropping more regularly.

The larger part of the answer is management. A dessert orchard thins its crop every year, chemically at petal fall and by hand afterwards, specifically to hold crop load below the level that suppresses return bloom. A cider orchard does not, because fruit size is irrelevant to the press and the labour cost cannot be recovered from a tonnage price. Cider trees are therefore routinely allowed to carry the crop loads that cause the problem.

The effect compounds across a district, because the trigger events are regional. A frost that takes out one orchard’s blossom takes out the county’s, and a whole growing region can be brought into phase by a single bad spring. That is why cider fruit supply swings from glut to shortage, why mills blend across years and hold juice or concentrate, and why single-variety vintage cider from a biennial cultivar is available in some years and not others.

Beyond the alternation itself, crop load changes the fruit. A heavily cropping tree produces many small fruit that share a fixed supply of photosynthate, and their soluble solids tend to be lower. A lightly cropping tree produces fewer, larger fruit, and while the sugar concentration may be higher, the phenolics laid down early in development are diluted through a larger volume.

So the on-year and the off-year do not simply differ in tonnage: they can differ in juice composition, in the same orchard, from the same trees. A maker pressing a biennial single-variety block is pressing a slightly different fruit in alternate years, and the resulting ciders are not identical even before anything happens in the cellar.

For the mill the consequence is logistical. A glut year overwhelms pressing capacity and forces fruit to be held, sometimes past the point where it should be; a light year leaves capacity idle and pushes buyers towards imported juice or concentrate. Neither is good for quality, and both are consequences of a physiological cycle in the orchard.

Thinning is the only reliable intervention, and it has to happen early: to affect floral initiation it must reduce the crop before initiation occurs, which means within a few weeks of bloom rather than in August. Chemical thinning at petal fall is standard in dessert growing and used occasionally on high-value cider fruit; hand thinning is effective and expensive.

Cultivar choice is the more realistic lever for a new planting. Choosing regular croppers for the bulk of the orchard, and confining strongly biennial cultivars to a proportion small enough that the block as a whole evens out, is how commercial cider plantings manage the problem without thinning.

Mixed planting has the same effect at a different scale. A traditional standard orchard containing a dozen cultivars of differing habits is far less prone to swinging as a unit than a monoculture block, which is one of several respects in which the old mixed orchard turns out to have been managing risk rather than merely being untidy.